The present invention relates to hot isostatic processing and in particular, to improvements in connection with the heating of powder metal workpiece preforms which are thereafter compressed to form a fused powder metal workpiece.
Hot isostatic processing of various materials is becoming increasingly important in providing articles with properties not otherwise attainable. Generally, such methods employ a compaction of a particulate preform of an article by uniformly compressing the preform at right angles to its exterior surface. This is achieved, for example, by employing as the pressurizing medium a fluid which inherently uniformly acts on the entire outer surface of the preform. This contrasts with uniaxial processes such as pressing, forging, drawing or the like, wherein the forces are substantially axially directed. Hot isostatic compaction produces improved physical characteristics, such as densification, strength, ductility and metallurgical and chemical changes not possible in the uniaxial compaction processes.
Generally, three types of isostatic processes have been utilized: cold isostatic processing (CIP), hot isostatic processing (HIP), and rapid omnidirectional compacttion (ROC). In the CIP process, a wide range of powder metals or ceramics may be isostatically compacted by a fluid medium at near ambient temperatures. Therein, for example, a flexible mold formed in the desired shape is filled with a powder material and placed in the cavity of a pressure vessel. The cavity is filled with fluid and pressurized up to 200,000 psi to achieve the compaction. The HIP process involves the pressurizing of an article such as a preformed unsintered part, a casting, a powder filled container, or the like, in a pressure vessel. An inert gas such as argon or helium is generally used. The vessel is gradually pressurized to about 15,000 to 30,000 psi while the part is heated at a predetermined temperature for a time sufficient to effect the desired densification and metallurgical and chemical changes. Thereafter, the vessel is gradually depressurized. While the process produces parts to final shape, avoids separate sintering and finishing operations and provides unique properties, the process cycle is extremely lengthy and costly.
In the ROC process, a powder metal workpiece preform is disposed in a ceramic shell or envelope, heated to a desired elevated temperature and then placed in a pressure vessel and pressurized to compact the preform. The ceramic shell acts as a liquid die material and, when placed in a suitable pressure vessel and pressurized such as by the use of a hydraulic ram, the ceramic material is rapidly pressurized in a short time interval. The preform is thus rapidly isodynamically pressurized and consolidated. The pressure may be rapidly released resulting in an extremely short cycle, namely seconds as compared to hours for the HIP process. Moreover, by heating the enveloped preform outside the pressure vessel, the size and construction of the pressure vessel is reduced and simplified. Heretofore, however, an external furnace has been necessary for the heating, and extended heating times have been required because of the low thermal conductivity of the ceramic enveloping the powder metal preform. The extended heating cycle is detrimental to the physical properties of the compressed workpiece. More particularly in this respect, the prior method and apparatus result in a compressed workpiece which has an undesirably low fracture resistance due to grain growth and particle separation resulting from extended heating.